Microelectronic imagers are used in digital cameras, wireless devices with picture capabilities, and many other applications. Cell phones and Personal Digital Assistants (PDAs), for example, are incorporating microelectronic imagers for capturing and sending pictures. The growth rate of microelectronic imagers has been steadily increasing as they become smaller and produce better images with higher pixel counts.
Microelectronic imagers include image sensors that use Charged Coupled Device (CCD) systems, Complementary Metal-Oxide Semiconductor (CMOS) systems, or other solid-state systems. CCD image sensors have been widely used in digital cameras and other applications. CMOS image sensors are also quickly becoming very popular because they are expected to have low production costs, high yields, and small sizes. CMOS image sensors can provide these advantages because they are manufactured using technology and equipment developed for fabricating semiconductor devices. CMOS image sensors, as well as CCD image sensors, are accordingly “packaged” to protect their delicate components and to provide external electrical contacts.
An image sensor generally includes an array of pixels arranged in a focal plane. Each pixel is a light sensitive element that includes a photogate, a photoconductor, or a photodiode with a doped region for accumulating a photo-generated charge. Microlenses and colored filter arrays are commonly placed over imager pixels. The microlenses focus light onto the initial charge accumulation region of each pixel. The photons of light can also pass through a color filter array (CFA) after passing through the microlenses and before impinging upon the charge accumulation region. Conventional technology uses a single microlens with a polymer coating, which is patterned into squares or circles over corresponding pixels. The microlens may be heated during manufacturing to shape and cure the microlens. Use of microlenses significantly improves the photosensitivity of the imaging device by collecting light from a large light-collecting area and focusing the light onto a small photosensitive area of the corresponding pixel.
The use of smaller-sized pixels is of increasing importance in microelectronic imagers because of the need to reduce the size of imager devices and increase imager resolution. Reducing pixel size, however, increases the problem of “noise” or background signals present in the image sensor readout when no light is incident upon the image sensor. This noise, referred to as “dark current,” is the result of electron activity within the substrate material carrying the image sensor. More specifically, the dark current is a result of thermally emitted charges being collected in the charge accumulation regions of the pixels. The magnitude of the dark current is dependent upon the image sensor architecture and the operating temperature.
One method of compensating for dark current is by masking off a set of pixels at a perimeter of the image sensor so that they are not exposed to light. Because the incident light is blocked from entering these pixels, the signal contained in these pixels is due only to dark current. These dark reference pixels are used as a “black level” reference for calibrating the image sensor output. One problem, however, is that it is difficult to accurately partition the dark reference pixels at the perimeter of the image sensor from the adjoining active pixels because the extremely small pixels are positioned very close together. For example, dark reference pixels in close proximity to the outer bounds of the image sensor can scavenge signal from the incident light because the dark reference pixels are not completely shielded. Accordingly, the measured dark current may not represent the true dark signal of the image sensor in such circumstances. Further, it is not desirable to move the dark reference pixels further outboard of the active pixels to avoid the problems with incident light because it will increase the size of the image sensor. Therefore, there is a need to enhance the performance and precision of packaging microelectronic imagers.